The field of the invention comprises apparatus and a method for imaging electrophotographic members by means of radiant energy devices such as lasers, the imaged electrophotographic members being thereafter used primarily for printing. In the case of lithographic offset printing the actual imaged member itself is treated to render toned and untoned parts hydrophobic and hydrophilic, respectively and the member comprises the printing plate without further processing. In other cases the toned electrophotographic member may be used as an information source by reading the images or projecting them if transparent or photographically reproducing them if desired. The preferred use of the invention is to make the printing plates either on transparent synthetic resin sheeting such as polyester or upon metal such as tin plated steel. Each of these substrates is coated with a type of photoconductive coating which will be described below.
In the printing industry, pictures, photographs and other images are usually reproduced by utilization of what has become known as half tone printing. In this process the original image or pattern of graphic material is photographed through a screen of crossed parallel and perpendicular lines to form an array of dots on the photographic film. Each dot in this array has a size and a spacing relative to the other dots which are produced which relates or is attempted to be related to the density of a corresponding incremental area in the original image which is being reproduced. Black and white graphics are photographed through the screen once while color graphics are photographed several times, once for each color which is to be printed. Each of the latter photographs is taken through the same screen using different color filters to separate the original image into its primary colors.
Using other photographic processes, these arrays of spaced dots are then transferred to metal surfaces to form the printing plates which will be installed in the printing press which is to reproduce the original pattern. The printing plates are required to print the respective colored images in precise registration on the receptor which is normally a web of paper and hence there will be as many impressions on a given area of the paper as there are color plates. The composite of these arrays of dots will produce a resultant array of dots many of which will overlie one another to give a color mixture attempting to reproduce the color of the originally photographed image as closely as possible.
If the screen through which the image has been photographed is sufficiently fine the human eye will not readily perceive the individual dots but instead will integrate the resulting array into shades of different density that simulate very closely the original image. An image which is a photograph, for example, is called a continuous tone image because there are no dots visible except to an extremely high power microscope and then what one sees are the grains of silver which are deposited side by side and without spaces normally present between them. A continuous tone image cannot be made into a printing plate because the ink will run and smear due to capillary action between side by side increments destroying the visual
In the case of black and white the use of half tone printing provides arrays of dots which give varying shades of gray between white where there are no dots and black where the dots are so close together and so large that they carry heavy coatings of ink in the press. In the case of color printing, the multiple impressions are required not only to provide the different shades of light and dark for information content but also to provide the multiple hues of color that are needed to attempt to reproduce the original continuous tone image.
The process of making printing plates of this type in accordance with conventional manual methods is labor intensive, time consuming and expensive. It requires considerable skill and much capital equipment for any large quantities of such plates as in the printing of periodicals, magazines, books and other largely circulated printed matter.
The process of making plates using the same techniques as have conventionally been utilized, that is, making the color separations and deriving the metal printing plates therefrom has been effected electronically in recent times. Photosensors are utilized to sense the intensity or density of incremental elements of a continuous tone image and then digital data are produced which are intended to represent the various densities of the so-called picture elements of the original image. This digital data is then used to reproduce the image as an array of dots on a printing plate with heat or light sensitive systems. These systems usually utilize laser beams to expose a master image on heat or light sensitive film or paper. The exposed film or paper is then processed to form the image on the printing plates. While these systems are not as labor intensive as the manual processes which have been mentioned above, they are nevertheless considerably more expensive than the process of the invention which will be described and they have other disadvantages.
The materials upon which such images are formed must be exposed or activated with a certain amount of radiant energy on an elemental area over a discrete period of time so as to form the image but not to burn through the material. This poses considerable problems. Imaging in this way may not be as time consuming as manual production of plates but is not rapid enough to perform either high speed or on-line imaging. Accordingly it is not utilized to any great extent for the manufacture of printing plates.
Another problem occurs when colors are overprinted by the use of the half-tone process. Printing several colors such as the primary colors to form the desired hues produces undesirable moire interference patterns. Moire patterns are produced when printed impressions from multiple screens having different numbers of lines per unit length are overlaid one on the other. Such patterns are also produced when printed impressions from screens having the same number of lines per unit length are overlaid slightly out of registration. These patterns are readily seen by the human eye as waves of light and dark lines in the printed image. In half tone processing the screen of lines through which the image is photographed reproduces the screen pattern in the printed image and also produces the moire pattern in the eventual printed image.
Moire patterns are not acceptable in quality printing and, in any kind of printing are annoying to the viewer besides distorting the reproduction of color.
The most common solution for this problem in manual half tone processing is to photograph each color separation with the screen lines arranged at a different angle relative to that of all other separations. Printing from the plates made with these separations then lays down the color patterns for the respective screens at the different angles chosen. Using this technique, the color separation is photographed and subsequently printed with the vertical lines of the screen arranged at an angle relative to a base line which coincides with or is parallel to the horizontally extending axis of the composite image to be printed. This is usually parallel to a horizontal edge of the paper or other stock which will carry the impression.
The colors of the composite image will comprise magenta, cyan, yellow and usually black in addition for enhancement. The angles which are conventionally used for printing these colors are, 90.degree. for the yellow, 75.degree. for magneta, 105.degree. for cyan and 45.degree. for black if it is used.
The solution of arrangement of the color separation screens at different angles is not a complete one because the eye will in many cases still be able to detect moire patterns. In addition, the technique results in the formation of small rosettes which can detract from the quality of a color image and can be quite annoying when they occur at critical locations on a given object.
When high quality printing is done as many as 18 different color separations may be used. Each color separation will have its own array of dots and will be processed to print at a different angle. Great care must be exercised to arrange the printing angles to reduce moire patterns and in addition the dot spacing must be chosen so that dot overlay is reduced. This is true when many impressions are to be made to achieve a particular color and is even more important where the inks tend to be opaque.
Electronic systems produce the color separations by sensing the original image through colored filters. The sensed picture elements are then digitized for use in forming the dot arrays. The sensed densities of the picture elements in each color separation are usually treated as steps of a gray scale, this scale extending from the least dense to the greatest. Each step of the gray scale is then used to form a particular pattern of printing dots on the printing plate. Each pattern of dots is equivalent in area density to the sensed intensity of a corresponding picture element of the original image. When the dot pattern is printed, theoretically an equivalent density of ink of each color then is transferred to the receptor or paper stock.
It should be noted that the dots formed in making the manual and electronic half-tone color separations are different. The dots formed in the manual process vary in surface area and spacing from the surrounding dots to produce the varying densities or shades of gray. Thus a light gray or weak density image is represented by small dots spaced a great distance from the surrounding dots. A dark gray or strong intensity image is represented by large dots, almost or actually touching each other. The dots formed electronically are generally fixed in size and spacing. Their size is usually determined by the material used and may be equal in size to the smallest dot formed in the manual process. The varying intensitities are formed or represented by the number of dots in a matrix of unit area. Thus a light gray image is represented by a small number of dots in each matrix or in one of several matrices. A dark gray image is represented by a large number of dots in a single matrix.
Moire patterns are also produced when printing from color separations which have been produced electronically. This occurs because of the regular formation of dots in each matrix, and the regular formation of the matrices relative to one another to form the image. Usually the dots of each matrix are formed in particular locations which are horizontally and vertically aligned. Each matrix is located so that its dots are in alignment with the dots of the preceeding and succeeding pixel. Even though the dots which form the image are integrated by the human eye, the alignment or registration of the dots becomes apparent in the form of moire interference patterns and/or rosettes.
The present invention eliminates these moire and rosette patterns by producing color separation plates which have irregular and varying patterns of printing elements to correspond to each step of the gray scale or hue of color to be printed. The elements of each imaged pixel which are intended to pick up ink are located so that they interleave and substantially overlap each other. Imaged pixels themselves are also located so that they interleave thereby avoiding possibility of vertical or horizontal alignment of printed elements.
The first step of the gray scale following pure white, which is represented by no printing elements in a pixel, is achieved by laying down a single printing element within the given pixel at any one of a plurality of different locations. This printing element is the result of removing from the pixel all other elements rather than actively putting an element in place in the pixel. Thus, if the pixel is designed to carry 19 elements; the process and apparatus remove 18 of the elements. Consider that the process of the invention is effected electrostatically and the pixel is formed by exposing a previously charged area on a photoconductive coating, if the presence of 19 rays of radiant energy directed at the pixel area will discharge the entire area leaving no charge at all, then the presence of 18 rays of radiant energy will discharge all but one element and that element will be tonable and can become a printing element. As seen hereinafter, the exposure of the pixel area does not require eighteen rays of radiant energy impinging at one time to expose eighteen incremental elements, but the principle should be understood that a printing element is produced by nondischarge of a charge from the photoconductive coating, the discharge of elements being effective to produce nonprinting increments of the pixel.
Further steps of the gray scale are achieved by forming additional printing elements in the pixel, these elements as a rule being adjacent and overlapped so that there is a single integral printing element in each pixel which will produce a printed formation that represents a particular density. Although it is easier to provide the single formation made up of several nondischarged areas within the pixel that are contiguous, it is feasible under certain circumstances to divide the nondischarged elements or areas into several, for example two, and achieve a subtle density step in the gray scale. In the formation of printing elements within the pixels, any one of a number of positions will be effected so that the changes of moire patterns forming become very low.
The use of a number of different patterns for the placement of the printing formations produced in each pixel ensures that the irregularity of the elements and the pixels will approach an almost random placement of the dots forming the printed result. The use of the word dots for the results of printing by the use of the invention is only for convenience since the printing formations are far from dots and are in no way the equivalent to the dots which are known in conventional or even electronic half tone printing which is known.
The variety of printing formations and patterns of their placement within the pixels is much greater than anything utilizing dots in the prior art. This is feasible because primarily there is a photoconductive coating for which the invention is especially advantageous which has a resolution that is much greater than other photoconductive coatings thereby enabling very small undischarged areas on its surface to be toned. These will form the printing elements or formations. This enables the use of a large number of elements to form the printing formation in each pixel without any detectable degradation of the resolution of the reproduced image. This advantage is one of the principal advantages of the invention and is in addition to the decrease if not elimination of moire interference patterns.
The preferred form of the invention utilizes 19 elements of discharge to form each imaged pixel thereby enabling several hundred different gradients of gray to be produced. With such pixels and multiple placement of the undischarged formations within the pixel over 30,000 different patterns can be produced giving a degree of density gradient never before achieved using conventional and known electronic methods with the conventional materials available at this time, and in addition decreasing the possibility of forming moire interference patterns or rosettes to a practical insignificance.
In summary, the printing formations for each degree of density of the gray scale are irregular in geometric configuration due to the manner of forming the same; the irregularity is increased because there are different locations within the pixel where these formations are placed; and the differing patterns of adjacent pixels as well as their interleaved arrangement produces more irregularity without sacrificing any of the resolution, gray scale or quality of the resulting printed image.
Typical prior art disclosing technique generally related to the subject matter of this invention comprise the following patents:
U.S. Pat. No. 4,084,259 PA1 U.S. Pat. No. 3,922,484
and some of the references cited in the said patents.